error messages

[Mike Reed]

Hi,
pretty simple question but I can't find the answer anywhere in the GCC
documentation or on the web/mailing lists.  I am getting an error from
the compiler like this:

$ g++  -c -o Viewer.o Viewer.cpp
In file included from Viewer.cpp:1:
Viewer.h:4:23: error: gtkmm/box.h: No such file or directory

I can see from the source that the #include occurs on line 4 of Viewer.h
but what is the 23?  I only seem to get this second number on "No such
file or directory" messages.  In the GCC documentation it only mentions
filename and line number being given, no mention of a possible second
number or what it is.

Just curious.

Thanks,
Mike.